1. Field of the Invention
The present invention relates to a process for the fabrication of cubic boron nitride from hexagonal boron nitride by using a binary alloy of Al and Mg as a catalyst under a high temperature and a high pressure.
2. Description of the Prior Art
Generally, the cubic boron nitrides having high hardness can be synthesized only by artificial methods. Though its hardness is not higher than that of diamond, its thermal stability is better so that it can be used such an abrasive as cutting tools for hardened steel.
Such cubic boron nitride can be synthesized under a high pressure and high temperature, like a diamond, either by a direct conversion method or by a indirect catalyst method. The former is a direct fabrication method from hexagonal boron nitride and the latter is used by adding the catalyst to the hexagonal boron nitride.
Among such two methods, the indirect method is more often used because of economical merit and lots of catalysts are known such as the alkali (Li), alkali earth (Mg, Ca), the alloy of these elements and the nitrides (Li.sub.3 N, Mg.sub.3 N.sub.2, Ca.sub.3 N.sub.2) of these elements. Among these catalyst materials, Li.sub.3 N, Mg.sub.3 N.sub.2, Mg are most often used (Wentorf, J. Chem. Phys., 34, 809 (1961)), and boro-nitrides such as Li.sub.3 BN.sub.2 of above elements (DeVries and Fleisher, J. Cryst. Growth 13/14, 38 (1972)) and Mg.sub.2 B.sub.2 N.sub.4 and Ca.sub.3 B.sub.2 N.sub.4 (Endo, Fukunaga, Iwata, J. Mat. Sci., 16, 2227 (1981)) are also often used.
These catalysts of alkali and alkali earth elements have excellent merits for the fabrication of cubic boron nitride. However, it is very difficult to treat them since its activity is high and they react actively with the moisture in the air.
There are many useful metal catalysts such as Al, Cr, Mn, Co, Ni-Al, Sn, Sb, Fe-Al, and Si-Al. However, the cubic boron nitride crystals produced by using these catalysts contain impurities.
As compound type catalysts, urea (Susa, Yogyo-Kyokai Shi, 86, 202 (1978), hydrides such as LiH (Vereschagin et al, Trans. Izv. Akad. Nauk SSSR. Neorg. Mat., 15, 256 (1979)) and fluorides such as NaF (Kobayashi, Mat. Res. Bull, 14, 1541 (1979)) are also known. However, although the cubic boron nitride crystals synthesized by using these catalysts are such a fine powder type, these catalysts are not proper to get large cubic boron nitride crystals.